Process of preparing porous sintered metal parts for metal plating



PROCESS OF PR EPARING POROUS SINTERED METAL PARTS FOR METAL PLATING Filed Aug. 4. 1955 v FLOW SHEET U NT D I LIQUID BATH OF HIGH TEMPERATURE M CROCRYSTA -LINE WAX WAX IMPREGNATED PARTS ALLOWED TOvgggL TO HARDEN IN N PA RTs PARTS TUMBLED IN BARREL WITH SLIGHTLY ABFLASIVE AGGREGATE PLUS GOLD DETERGENT l WASH WITH GOLD-WATER COLD PREPLAT NG CLEAN ERs MAY BE ACID OR BASIC cou: ELECTROLYTIC PLAT'ms BATH INVENTORS MILTON F. sm'ru A BY HENRY F. LATVA.

RNEYS United States Patent PROCESS OF PREPARING POROUS SINTERED METAL PARTS FOR METAL PLATING MiltonF. Smith, Birmingham, and Henry. F. Latva, Dearborn, Mich., assignors, by mesne assignments, to The Bunting Brass and Bronze Company, Toledo, Ohio, a corporation of Ohio Application August 4, 1 955, Serial No. 526,541 3 Claims. (Cl. 117-102) This invention relates to aiproce ss of preparing porous sintered metal parts for. plating metal films' thereon, and more particularly to such a process in Whichavoidance of surface corrosion after plating is an important factor.

The process of this invention embodies generally the following steps: For example, taking a sintered metal part of moderate density, containing from 10 to 40% voids, and soaking the part in a liquid bath of a high temperature microcrystalline wax; removing the part from the bath, after a period of time sufiicient to substantially impregnate the part and fill most all of the voids, and draining oif the'liquid wax, whereby a thin, but often irregular, surface film of wax coats the part; tumbling the part in a barrel containing a slightly abrasive aggregate and a liquid detergent solution for a period of time suficient to mechanically remove the surface film of wax, but not to penetrate the voids, leaving the surface of the part in a burnished condition substantially free of pits or nicks; and finally, washing and rinsing the parts clean of surface adherent impurities. Following this processing the sintered metal part is ready for pro-plating cleaning operations and subsequent plating according to standard commercial practices. One limitation, however, must be exercised both in the process and in the pre-plating and plating treatment, namely that the temperatures of liquid solutions, rinses, washes and solid matter be maintained substantially at room temperatures, or at relatively cold temperatures, in order that the wax remain firm and hard. It is an essential feature of the process that only the outer overlying film of wax be removed at any time. When the wax level goes below the surface of the metal, i.e. sinks into the voids, a certain amount of metal skin below the surface is exposed to the pre-plating and plating treatment solutions. Pre-plating washes and cleaning solutions are either acidic or caustic (basic), and, sometimes, both types of cleaning solutions are used to prepare a part for plating, depending upon the nature of the metal surface condition. These strongly corrosive solutions attack the metal surface and, in the case of sintered porous parts, the liquid solution enters the pores and voids to remain there and continuously attack and corrode the metal, even after plating treatment, resulting in discoloration and corrosion bloom.

In. the prior art, sintered porous metal parts have been impregnated with waxes, tung oil, plastics, oxides andmany other impregnants which harden or polymerize over a short or long period of time to fill the voids in the part. An excess of the impregnant, as a surface coating or film, remains on the part. Before satisfactory metal plating can be accomplished, this film must be removed. In the past, this removal step has been performed either mechanically or chemically. Mechanical removal has taken the form of tumbling the parts in a barrel with burnishing steel balls, or a smooth aggregate. I his. form of removal is fairly'incomplete' and unsatisice vide basins for accumulation of the corrosion-starting and accelerating matter in the pre-plating and plating solutions.

The chemical, or solvent, process removal of the over layer of impregnant is unsatisfactory because it is so very diflicult, if notimpossible, to determine just how long the dissolving action should proceed. Some parts are more heavily coated than others, and portions of a single part may be more or less heavily coated with the impregnant than another portion of the same part. The result is that some parts or portions of a part have the impregnant removed from .thel voids below the surface of the sintered part. 'This at once provides a storage area for the corrosive pre-plating washes and solutions, resulting in subsequent corrosion of the metal surface. 7

For these reasons, a new approach was developed in the instant invention, based on an analysis of the defects of the prior art. It is therefore an object of the invention to provide a process of preparing porous sintered metal parts containing voids therein, in which the voids are filled with an impregnant to excess. Another object is the provision of a step in which only' the surface coating or overlayer is removed. A further object is to provide for such removal by mechanical means. Still another object is to provide a mechanical means for removal of the excess film coating without damaging the part and the surface of the part. Still another object is to provide a mechanical removal of the excess overlying impregnant without entering the voids or pores of the sintered part, by an abrading action of an aggregate in the removal operation, and by a slight burnishing of the metal surface of the part.

These and additional objects. and features of the invention will become more apparent from the description given below, in which the terms are used for purposes of description and not of limitation.

The figure is a flow sheet illustrative of the process of preparing sintered metalparts for metal plating.

The process involves a porous powdered metal part. This part may be of any suitable or desirable shape and size. It may be made of any metal, metal alloy or combination which can be briquetted, sintered and surface plated with another metal, according' to current commercial plating practices. Many times, powdered non-metals may be combined in smaller or greater proportions with powdered metals, depending ,upon the nature of the particular application. For the purpose of this disclosure, such combinations are considered as porous metal parts. These non-metals may come from a group including metal oxides, clays, and similar ma terials, but are not necessarily limited to such group; Because the number and range of such non-metals as have been or may be combined with metals is so great, and because persons skilled in the art to which this invention pertains are very familiar with such combinations, it is not considered necessary to specify any particular non-metallic material in combination with any particular metallic material.

Examples of metals that are commonly used in powdered metal structures include copper, tin, nickel, chromlum, zinc, aluminum, ironand steel, either alone or in alloyed forms.

A-n exampleofa powdered metal part which has been successfully processed in accordance with this invention is an iron bushing of 1" outside diameter, inside diameter, and long. The piece is briquetted of powdered iron and sintered at approximately 2000 F. for one hour in a reducing or neutral atmosphere. The part is very porous, containing 20 to 25% voids.

At this stage, if the part were put directly into the pre-plating washes or cleaning solutions, it would absorb a considerable amount of the solution, which would result in a substantial corrosion of the part. Cold water or hot water rinses do not remove the corrosive solutions from the voids. For this reason, an impregnant is used to fill the voids. A microcrystalline wax, taken from the petroleum group, having a melting point of 190 F. has been found very satisfactory. The wax is placed in a vessel and heated to about 250 F. Since most of the processed parts are relatively small, they are loaded into a wire basket and immersed in the liquid wax bath. The wax impregnation step lasts for about 20 minutes. At the end of this period, the voids in the porous parts are substantially filled. The parts are then removed from the wax dip bath, and the excess wax drips off leaving a thin film coating on the parts.

Waxes of the microcrystalline group having melting points in the range upwards of 145 F. have been found satisfactory. The wax film coating, when below the melting point or range, is substantially hard and firm. The wax film should tend to chip, break or abrade off when struck by a hard aggregate in the tumbling and cleaning operations which follow.

Following the wax dip, the parts may be subjected to a sizing operation, in which the final dimensions prior to plating are obtained. This operation tends to remove a considerable amount of the excess wax on the surface, but not all of it. A thin wax film usually remains and must be removed before the plating step takes place.

To remove the wax film coating from the surface of the sintered parts, the parts are placed in a tumbling barrel containing an aggregate and a liquid soap or detergent solution. Most tumbling barrels are rotary in motion, i.e. they revolve on a shaft, generally horizontal in position, so that the parts and the aggregate roll over and over each other. The aggregate is selected from a group of materials that are generally harder than the part being treated, so that the tumbling action results in an abrasive and/or burnishing action. The tumbling is done in a wet liquid solution of a soap or detergent, the function of the solution being to hold the wax and other impurities being removed from the parts by the aggregate in suspension, so that these undesirable materials will not become redeposited upon the parts. Any of a large number of such soaps or detergents may be used, since these materials are commercially and readily available from industrial supply houses. Any person skilled in the art to which the invention pertains can make a suitable selection for this operation.

The aggregate may be taken either from a group of natural rock or from synthetic materials. Such natural materials as granite, in rock or chip form, would be suitable. Granite is often found naturally mixed with softer rocks or stone, forming a suitable aggregate. Such synthetic aggregates as the porcelains, ceramics, and fused or bonded aluminum oxides may also be used. The size and shape of the aggregate depend upon the size and shape of the parts being treated. Where the work has a hole or bore on which a deposit of wax has been made, the aggregate should be selected to pass through such opening so as to act upon its surface. Most granite rock chips have sufficient edges and rough surfaces to produce an abrasive action upon the surface of the waxed metal parts. The result of this abrasive action is to scrape off the excess film of wax. Where the aggregate material does not have too high an abrasive effect, the aggregate may be obtained in shapes that have more or less sharp edges, such as a triangular shape. Thus, the

same desirable result may be obtained. Another result of the action of the aggregate is to burnish the surface of the work, i.e. the edges of metal surrounding the pores or voidsnow filled with waxare rolled over, so as to produce a smooth, deburred surface. Rough edges and burrs on the parts are smoothed down by this tumbling action so that the parts when plated are completely finished, ready for use.

Tumbling speeds and time are factors which may be somewhat varied. However, very satisfactory results have been obtained by barrel tumbling for 2 or 3 hours at moderate to slow speeds, the temperature of the liquid detergent solution being at substantially room temperature.

After the excess film coating of wax has been removed by this step in the process, the sintered metal parts are washed in a cold water rinse and are ready for the preplating cleansing operations. The voids or pores of the metal parts are now filled with hardened wax, insoluble in the cleansing and wash solutions, and only the edges of the metal, surrounding the filled voids, in burnished condition, are presented for cleansing and subsequent 7 metal plating.

Since the parts do not absorb any of the highly corrosive cleansing agents, they are easily cleaned and ready for plating.

In the plating step, it has been found that when plated the parts present a substantially continuous surface of plated metal. This appears to be due to the fact that the metal is supported upon the wax filler in the voids, which is at metal surface level. The plating metal, so to speak, bridges the voids, upon the wax. The result is a corrosion-free, metal-plated sintered metal part.

The metal plates upon the cleansed part generally deposits as a continuous overlayer or film, bridging the wax or filler containing voids. The metal plated film is usually thin in cross-section, one government specification calling for a .0003" thick layer of cadmium. Of course, thicker metal coatings may be plated upon the parts prepared according to this invention, if desired or required, the character of the plated overlayer forming no direct feature of this invention.

The parts may be plated with such metals as cadmium, zinc, chromium, copper, nickel, tin, and other metals deposited by the plating processes on metal.

Of course, microcrystalline wax, as here described, is the preferred material for filling the voids of the sintered metal part, but other waxes or materials may also be employed as fillers so long as the surface overlayers are removable by mechanical means down to the metal surface, and so long as such removal does not create basins in the filled voids for accumulation of corrosion-generating agents during pre-plating or plating operations.

The step of removing the surface overlayer of wax, so that the metal and wax filler in the voids will present a continuous surface, without basins for accumulation of electrolyte or pre-plating solutions, may also be accomplished by such mechanical operations as scraping the metal surface with a cutting tool, a reamer, an abrasive wheel, or similar devices, by which the metal and wax surface levels become uniplanar. In this way, the sintered porous metal part is provided with a surface that supports a metal plating film without any accumulation of corrosion-generating material under the plating film. Thus, corrosion cannot start from within the porous part, and the relative functional value of the part is increased many fold. In fact, if desired, the part may be prepared for plating in only a single area, or in selected areas, by removing the wax overlayer to the metal surface level by any of the above described means, in the desired areas. After pre-plating cleansing and metal plating, the remaining wax overlayer may be stripped by the usual means. In some cases, these means would be wax solvents, which are readily available and known to persons skilled in the art to which the invention pertains. In other cases, it may be preferable to remove the excess wax overlayer by mechanical means, either identical with or similar to those given above. The selection of means for removing the wax overlayer would be dependent upon the particular application involved-the part, its function characteristics and configuration. Thereafter, the part should be washed in a slight detergent solution to cleanse the part of the particles of wax removed by the mechanical treatment.

It will be noted that the defects and failures of the mechanical operations previously used-shot blasting, shot peening, wire brushing, etc.'are avoided by the instant invention through the use of an aggregate in a wet tumbling operation wherein the surface overlayer of a wax filler is abrasively removed, but the impregnating wax filling the voids remains at metal surface level. Thus, the work has a continuous level surface which prevents absorption or accumulation of corrosive agents, and accepts metal plating more easily and satisfactorily.

Having described our invention in its simplest terms, it is to be understood that the features of the process may be varied in greater "or lesser degree without departing from the invention orthe scope of the appended claims. i

We claim:

1. The methodof preparing a porous sintered metal article for plating which comprises immersing the article in melted microcrystalline wax which is solid at room temperature to fill the surface pores and cover the sur- References Cited in the file of this patent UNITED STATES PATENTS 2,187,086 Koehring Jan. 16, 1940 2,197,376 Emmert Apr. 16, 1940 2,222,410 Calkins Feb. 8, 1940 2,318,581 Balz et a1. May 11, 1943 2,461,410 Clark Feb. 8, 1949' 2,540,457 Rice Feb. 6, 1951 2,652,624 Guinee Sept. 22, 1953 2,663,928 Wheeler Dec. 29, 1953 2,683,343 Gillette et a1. July 13, 1954 OTHER REFERENCES Pratt: Precision Metal Moulding, January 1953, pp. 447-49.

Rushbrook: Electroplating and Metal Spraying, vol. 7, #3, March 1954, pp. 92-96 

1. THE METHOD OF PREPARING A POROUS SINTERED METAL ARTICLE FOR PLATING WHICH COMPRISES IMMERSING THE ARTICLE IN MELTED MICROCRYSTALLINE WAX WHICH IS SOLID AT ROOM TEMPERATURE TO FILL THE SURFACE PORES AND COVER THE SURFACE OF THE ARTICLE, COOLING THE ARTICLE TO HARDEN THE FILLER MATERIAL, TUMBLING THE ARTICLE IN A BATH OF DETERGENT MATERIAL AT A TEMPERATURE AT WHICH SAID FILLER MATERIAL REMAINS HARD, SAID BATH CONTAINING CHIPS OF NATURAL OR SYNTHETIC ROCK-LIDE MATERIAL, AND CLEANING THE ARTICLE IN A PRE-PLATING SOLUTION AT A TEMPERATURE AT WHICH SAID FILLER MATERIAL REMAINS HARD. 